1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image to a receiving material upon application of heat and pressure thereto, and to an image forming apparatus, such as an electrophotographic copier, printer, facsimile machine or the like, having the fixing device.
2. Discussion of the Related Art
Conventionally, electrophotographic image forming apparatuses, such as electrophotographic copiers, printers and facsimile machines, having a fixing device in which a toner image formed on a recording sheet is fixed thereto upon application of heat and pressure, are known.
Such a fixing device tends to induce toner scattering, such that when a recording sheet bearing an unfixed toner image thereon enters a fixing nip of the fixing device, toner particles constituting the unfixed toner image are electrostatically scattered and scattered due to uncontrolled movement (swinging or the like) of the recording sheet. Particularly when a recording sheet bearing an unfixed toner image thereon is vertically fed to a fixing device, toner particles constituting the unfixed toner image often scatter at the entrance of the fixing device, thereby contaminating neighboring components such as a heating roller and/or a pressing roller of the fixing device, resulting in formation of an abnormal image with the background and/or backside soiled with toner transferred from the rollers. Jamming of a recording sheet in the fixing device because the sheet is adhered to the contaminated roller often results as well.
The abnormal image problem will be described in detail.
Specifically, electrophotographic image forming apparatuses typically perform an image forming operation such that after a toner image formed on a photoreceptor is transferred onto a recording sheet, the toner image is fixed thereto by a fixing device.
The fixing device typically has a heating roller having a heater therein or an induction heater arranged outside, and a pressing roller rotating contacting the heating roller to form a fixing nip so that when a recording sheet bearing an unfixed toner image thereon passes through the fixing nip, the toner image is fixed to the recording sheet.
In this regard, the unfixed toner image is contacted with the circumferential surface of the heating roller. Even when the circumferential surface of the heating roller has good releasability initially, foreign materials such as toner particles (such as small toner particles and toner particles finely pulverized in a developing device) and paper dust adhere thereto after long repeated use. In attempting to prevent adhesion of such foreign materials, a cleaner such as cleaning rollers, cleaning blades, and cleaning pads is typically contacted with the circumferential surface of the heating roller. However, even in this case, foreign materials such as toner particles adhere to the cleaner, thereby degrading the cleanability of the cleaner. Therefore, the cleaner has to be frequently replaced with a new cleaner, thereby complicating maintenance, and increasing running costs.
In addition, toner particles scattering at the entrance of the fixing device can easily adhere to the pressing roller, and the toner particles adhered to the heating roller are transferred onto the pressing roller. Therefore, the pressing roller is also typically made of a material having good releasability.
In attempting to solve the toner scattering problem, a technique such that a voltage having a polarity opposite to that of the toner (developer) used is applied to a cleaning roller contacted with a heating roller is proposed. However, the technique has several drawbacks. For one, the cleaning roller serving as a biasing roller is always contacted with the heat roller, the surface of the heating roller is damaged, and the heating roller is contaminated worse than in a case in which the cleaning roller is not provided because foreign materials adhered to the surface of the cleaning roller rub the surface of the heating roller. Moreover, foreign materials other than charged toner particles cannot be removed from the surface of the heating roller by the cleaning roller, necessitating provision of another cleaner to clean the surface of the cleaning roller.
In addition, an unfixed toner image on a recording sheet is fed to the entrance of a fixing device in an unstable state, insofar as the toner is adhered only electrostatically to the sheet. In addition, with recent miniaturization of electrophotographic image forming apparatuses, a recording sheet bearing an unfixed toner image thereon is typically fed vertically to a fixing device, making the toner image even less stable. When a recording sheet bearing the toner image is swung in a sheet feeding passage leading to the fixing device, or strongly strikes the entrance of the fixing device, or the electric potential of the toner image changes because of being contacted with a guide member guiding the sheet to the fixing device, the toner image on the sheet may be mechanically or electrostatically scattered, thereby contaminating various parts of the fixing device and image forming apparatus, with the undesirable results described previously.
For these reasons, there is a need for a fixing device which hardly forms abnormal images caused by the above-mentioned mechanical and electrostatic toner scattering problems.